This invention relates to a vent type injection molding machine.
To have better understanding of this invention, a prior art vent type injection molding machine will firstly be described with reference to FIG. 4. The injection molding machine shown therein comprises a screw 1 divided into a first stage a and a second stage b. The first stage a comprises a feed zone F, a compression zone C and a metering zone M starting from a hopper 2. In the same manner, the second stage b also comprises a second feed zone f, a second compression zone C' and a metering zone m. The screw 1 is contained in a heating cylinder 3 heated by electric means, for example, and provided with a nozzle 4 at its outer or front end. A thermoplastic resin supplied to the heating cylinder 3 through hopper 2 is transferred to nozzle 4 by the rotation of screw 1 while being melted or plasticized by the heat of the heating cylinder 3. The quantity of the molten resin is metered by the metering zone m. The heating cylinder 3 is provided with a vent opening 5 at an intermediate point for venting to the outside excessive moisture and volatile components contained in the plasticized resin. The position of the vent opening 5 is selected such that the vent opening would lie in the feed zone f after the screw 1 has plasticized and metered the resin and has retracted over stroke S from the foremost position shown in FIG. 4 to the rearmost position. When the position of the vent opening 5 is selected as above described, since the resin pressure which was increased in the first stage a rapidly decreases to the atmospheric pressure in a region A shown by the resin pressure curve shown in FIG. 4, venting is effected through the vent opening 5.
With the prior art injection molding machine shown in FIG. 4, as the screw 1 has been retracted by a metering stroke S so that the vent opening comes to the outside of the atmospheric pressure region A, the molten resin would flow out to the outside of the cylinder 3, so that a so-called vent-up phenomenon occurs. To prevent this vent-up phenomenon it is necessary to make longer the second feed zone f to be longer than stroke S which not only increases the length of the screw but also the total length of the injection molding machine. Consequently, the total weight and the floor space of the machine increase, which is not economical.